Following the actual granulation process, some plastic granules are subjected to heat treatment to change the structure of the granules. For example, polyethylene terephthalate granules also referred to here by the abbreviation PET granules, from the granulation process, are initially obtained in an amorphous state. The amorphous granules are converted into an at least partially crystalline state in a subsequent stage of treatment, known as crystallisation. Pronounced orientation of the molecular chains occurs in this process. In practice, degrees of crystallisation of around 30 to 50% are achieved in the crystallisation of PET granules.
Temperature control plays an important role in crystallisation. Firstly, the amorphous granules must be brought to or kept at a suitable reaction temperature. Secondly, some granules, for example PET or PU granules, tend to conglutinate in the transitional phase. PET granules become sticky even below the reaction temperature required for crystallisation, which lies between approximately 80 and 170° Celsius. The granules must be agitated during crystallisation to prevent their agglomeration. Their tendency to conglutinate decreases as the degree of crystallisation increases.
Whilst the amorphous granules are subjected to a crystallisation stage at temperatures below 80° Celsius in conventional granulation processes while being heated and powerfully agitated, it has recently been proposed, in connection with underwater granulation methods, that hot, pre-dried granules be exposed to vibratory excitation during an immediately subsequent crystallisation process. The vibratory excitation prevents conglutination of the plastic granules. The process heat stored in the granule particles may be exploited simultaneously for crystallisation, so that no additional heat is required in underwater granulation methods. In contrast, granules which have, for example, been stored in a silo in the interim, must first be cooled to avoid conglutination and subsequently be reheated for crystallisation.
In WO 2005/044901, a method is described for the thermal retreatment of PET granules in which the amorphous granules are fed through a vibrating trough for crystallisation. Said trough is divided into multiple consecutive chambers by intermediate walls in the form of weirs. The trough is vibrated in its direction of longitudinal extension by vibration motors. In this way the granules are conveyed along the trough by the vibratory excitation. The intermediate walls dam the flow of granules, causing longitudinal turbulence. At some point the individual granules spill over the intermediate weir walls and reach the granule outlet. The vibrating trough consequently has the same effect as a series of agitation vessels. Although this can prevent conglutination of the individual granules and achieve crystallisation, this configuration has the disadvantage of a relatively broad granule dwell time spectrum.
The use of a vibratory conveying device for crystallisation of PET granules is also known from DE 10 2004 050 356 A1. Vibration is excited in the direction of conveying in this case too. Compared to WO 2005/044901, the granules have a relatively short dwell time of 20 to 90 seconds in the vibratory conveyor device. Although a better dwell time spectrum can be achieved here, the path followed by the individual granules is relatively short, restricting their potential for interaction with other granules. This is unfavourable for equalising the temperature between the granules. Granules which predominantly traverse the trough on the surface of the flow of granules experience a different temperature gradient to those closer to the floor.
Achievement of a good dwell time spectrum is the intention of a vertical shaft reactor, as disclosed in DE 100 49 263 A1. However, no vibratory excitation is provided in this case.